Universal Mobile Telecommunications System (UMTS) is a 3rd Generation (3G) mobile communication system employing Wideband Code Division Multiple Access (WCDMA) technology standardized within the 3rd Generation Partnership Project (3GPP). In the 3GPP release 99, the radio network controller (RNC) controls resources and user mobility. Resource control includes admission control, congestion control, and channel switching which corresponds to changing the data rate of a connection.
The Long Term Evolution (LTE) of UMTS is under development by the 3rd Generation Partnership Project (3GPP) which standardizes UMTS. There are many technical specifications hosted at the 3GPP website relating to Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN), e.g., 3GPP TS 36.300. The objective of the LTE standardization work is to develop a framework for the evolution of the 3GPP radio-access technology towards a high-data-rate, low-latency and packet-optimized radio-access technology. In particular, LTE aims to support services provided from the packet switched (PS)-domain. A key goal of the 3GPP LTE technology is to enable high-speed packet communications at or above about 100 Mbps.
FIG. 1 illustrates an example of an LTE type mobile communications system 10. An E-UTRAN 12 includes E-UTRAN NodeBs (eNBs) 18 that provide E-UTRA user plane and control plane protocol terminations towards the user equipment (UE) terminals 20 over a radio interface. An eNB is sometimes more generally referred to as a base station, and a UE is sometimes referred to as a mobile radio terminal or a mobile station.
Each base station transmits a signature sequence over an entire cell area for the UE terminals to detect and measure. Measurements performed by the UE terminals on the received signal strength of different base station signature sequences are used in most radio communication systems (e.g. GSM, WCDMA, LTE, WCDMA-2000 etc.) to perform, e.g., initial cell selection and handover decisions. A signature sequence in WCDMA includes a particular scrambling code that is applied to the common pilot channel transmitted from each NodeB. The WCDMA standard specifies 512 unique scrambling codes with 512 corresponding MCIs. In LTE, a signature sequence is two-dimensional and is generated as a symbol-by-symbol product of a two-dimensional orthogonal sequence and a two-dimensional pseudo-random sequence. In total, the LTE standard defines 510 such unique signature sequences with 510 corresponding MCIs. In LTE, UEs measure the signature sequence for neighboring cells to determine a reference symbol received power (RSRP), and these RSRP measurements are used when performing initial cell selection for UEs to “camp” on as well as when performing handovers of UE connections.
Ideally, the signature sequences that a single UE can detect are uniquely mapped to a particular base station. But in most radio communication systems, the number of unique signature sequences that a particular standard specifies is less than the number of base stations in the system. The number of signature sequences is limited because transmission of a signature sequence is associated with a radio resource cost, i.e., power, bandwidth, code space, frequency space, or time, and that cost increases with the number of unique signature sequences for which the system is designed. Another reason why the number of signature sequences is limited relates to the UE mobile stations frequently reporting measurements related to the different signature sequences back to the radio network, e.g., to the serving base station. A UE may report several such measurements several times per second, and therefore, it is desirable that such measurement reports can be encoded with fewer bits to reduce the impact of those reports on the limited radio bandwidth.
In light of these considerations, a one-to-one mapping may be established between a signature sequence transmitted by the base station and a measurement cell identity (MCI) used by the UEs in the encoded measurement reports. The term MCI is used here as a convenient way of specifying which particular signature sequence a given base station is transmitting. An MCI may be viewed as an index that permits the UE to determine the corresponding signature sequence.
UEs continuously monitor system information as well as the signature sequences broadcasted by base stations within range to inform themselves about “candidate” base stations in the service area. When a UE needs access to services from a radio access network, it sends a request over a random access channel (RACH) to a suitable base station, typically a base station with the most favorable radio conditions. As shown in FIG. 1, the base stations are interconnected with each other by means of an X2 interface. The base stations are also connected by an S1 interface to an Evolved Packet Core (EPC) 14 which includes a Mobility Management Entity (MME) by an S1-MME and to a System Architecture Evolution (SAE) Gateway by an S1-U. The MME/SAE Gateway is as a single node 22 in this example. The S1 interface supports a many-to-many relation between MMEs/SAE Gateways and eNBs. The E-UTRAN 12 and EPC 14 together form a Public Land Mobile Network (PLMN). The MMEs/SAE Gateways 22 are connected to directly or indirectly to the Internet 16 and to other networks.
One important focus area in LTE/SAE standardization work is to ensure that the evolved network is simple to deploy and cost efficient to operate. The vision is that the evolved network will be self-optimizing and self-configuring in as many aspects as possible. A self-configuration process is one where newly-deployed nodes are configured by automatic installation procedures to get the necessary basic configuration for system operation. A newly-deployed base station typically contacts a central server (or several such servers) in the network and obtains configuration parameters needed in order to start operating. Self-optimization is a process where UE and base station measurements and performance measurements are used to automatically “tune” the network. One example is automating neighbor cell lists, and one non-limiting way of automatically building neighbor cell lists is described in commonly-assigned, U.S. patent application Ser. No. 11/538,077, filed on Oct. 3, 2006, and published as US 2007/0097938, the contents of which are incorporated herein by reference. In GSM and WCDMA, base stations send neighbor cell lists to connected UEs so they have a defined set of cell broadcasts to measure (e.g., the signal quality or strength of those broadcasts) to permit determination of which if any neighbor cells is a suitable candidate for handover. In an LTE system, neighbor cell relation (NCR) lists are also used in the eNBs to set up connections over the X2 and/or S1 interfaces.
An area potentially advantageous for self-configuring is automatic assignment of shorter measurement cell identities (MCIs) to base stations. Shorter cell identifiers like an MCI used in the UE measurement reports frequently transmitted to the network reduce the amount of radio resources consumed. The shorter cell identifiers are therefore sometimes referred to here as reporting cell identifiers. In addition to the short MCI, each cell is associated with a longer cell identity that uniquely identifies the cell within the public land mobile radio network (PLMN) to which the cell belongs. A non-limiting example of such a longer identifier is a cell identity on the PLMN level (CIPL).
With a limited number of MCIs or other reporting cell identifiers, some MCIs are likely to be reused in larger networks, which means network planning is needed. Today, such planning is typically done manually. For example, when planning in an LTE RAN, each cell in the network is assigned an MCI, and the planner tries to distribute the MCIs so that neighboring cells do not have the same MCI. But such attempts may not always be successful. This is true even if this operation is to be performed automatically using a suitable allocation algorithm implemented on a computer. An automatic MCI allocation algorithm should preferably also be capable of assigning MCIs in difficult networks deployments, e.g., networks with a large number of home base stations over which the network operator has little control.
In order to perform a handover in LTE from a source cell to a target cell, the two involved cells must first set up a neighbor cell relation (NCR). The NCR contains at least the MCI (or other short cell identifier) and the CIPL (or other longer cell identifier). The NCR may also include connectivity information such as the IP address of the corresponding cell, information about the configurations of the X2 and S1 interfaces, and parameters needed for different radio resource management control algorithms, such as handover thresholds. Information about the radio access technology (RAT) (e.g., LTE, WCDMA, or GSM) as well as other capabilities of the target cell may also be included in the NCR.
Building the NCR list in each base station can be done automatically. Each time a new base station is deployed, it contacts a central server in the network and that server assigns the base station with a CIPL and an IP address. The base station may begin operation with an empty NCR list, and each time it receives a measurement report from a served UE that contains a MCI that is not included in the NCR, the base station asks the UE to obtain the CIPL of that corresponding (non-serving) base station. In LTE, the CIPL is broadcasted (infrequently) on the broadcast channel (BCH) which allows the UE to detect the corresponding CIPL of the non-serving base station and report it back to the serving base station. The serving base station can then contact the central server to obtain the remaining NCR information corresponding to that non-serving base station.
When a base station has two neighbors with different CIPLs but with the same MCI, there is an MCI “collision” or conflict. As a result, one or more cells must change its old colliding MCI to a non-colliding MCI. To make this change requires closing down the cell, reconfiguring the new MCI value, and then restarting the cell. Alternatively, the cell may just change the MCI without closing and restarting, which means that all the UEs currently “camped” on that cell loose synchronization disturbing all active UE communications in that cell. Those disturbed UEs must perform new cell searches likely resulting in at least most of them selecting that same cell and performing a random access attempt. Such a mass random access is problematic because the typical random access channel is not designed to handle a large number of simultaneous access attempts. Alternatively, those UEs could select another, less satisfactory cell.
Another problem with such MCI collisions is that all neighboring cells to the cell with the new MCI no longer have correct and current information in their respective neighbor cell relation (NCR) lists. Consequently, when those neighboring cells receive measurement reports from the UEs using the new MCI, the neighboring cells must then re-establish their relationship to the cell with the new MCI. Until then, the neighboring cells can not order any UEs to perform a handover to that cell, which could result in dropping those calls that need handover being dropped.
MCI collisions will cause significant problems when new home or other base stations are set up without any coordination in a densely populated area (e.g., Manhattan). Each time a consumer sets up a home base station or moves the location of that home base station, there is a high likelihood of many MCI collisions because the network operator is not in control of that base station set up or movement and therefore can not do the cell planning/coordination needed to avoid MCI collisions. During the “roll-out” phase of a new network, new cells will be added, and MCI collisions are also likely to occur as a result. “Relay” base stations may also be installed in moving vehicles like cars, buses, and trains. Because these base stations move around, frequent MCI collisions may be experienced. Also, other autonomous changes in a self-organizing network, like adjustments in power levels or in antenna tilt, may cause MCI collisions to occur.
Most of the MCI collisions manifest themselves as ambiguities in NCR lists, i.e., two cells with different CIPLs but with the same MCI are listed in the NCR list. However, it is also possible that a UE experiences acceptable radio conditions to the serving cell and the candidate cell and that both use the same MCI. In that case, the UE may not be able to report the weaker cell as a candidate cell to the serving cell because the weaker cell is directly interfered by the serving cell on the same signature sequence. To intuitively explain why this is the case, consider two different scenarios.
In the first scenario, only the serving cell is transmitting a signature sequence. The UE searches for this signature sequence by cross-correlating a received signal with the same signature sequence. If the received signal contains the corresponding signature sequence, then the cross correlation output contains a distinct peak. The location of the peak provides the UE with time synchronization, and the amplitude of the peak is proportional to the signal strength of the corresponding cell. But due to multi-path propagation in the radio channel, the UE receives multiple copies of the transmitted signature sequence that have different delays, amplitudes, and phase shifts. There may for example be a direct signal path from the transmitter in the cell and the UE and an indirect reflected signal path that arrives at the UE sometime later. In this case, the cross correlation operation in the UE produces two distinct peaks for the same signature sequence, one per radio path. The UE uses this correlation peak information to perform channel estimation and equalization.
Now consider a second scenario without multi-path, but with two different cells transmitting the same signature sequence. The transmitted signals from the two cells will arrive at the UE with slightly different delays, amplitudes, and phase shifts. The UE performs a cross correlation with the received signal and the output contains two distinct peaks for the same signature sequence, but in this case, one peak per cell. What these two scenarios demonstrate is that the UE can not differentiate between normal multi-path in the radio channel and the same signature sequence being transmitted from two different cells.
Thus, it is desirable to provide an automated approach to resolve cell identifier collisions in an efficient fashion as well as to be able to disseminate changed or new cell identifiers in a seamless, automated, and coordinated fashion.